We have discovered that RALBP1 (RLIP76), a Ral-binding Rho-GAP protein is a transporter of glutathione-conjugates as well as structurally unrelated toxic xenobiotics. Preliminary studies in RALBP1-/- mice show marked radiation sensitivity in these animals compared with the RALBP1+/+ wild-type C57 mouse, with 0/6 RALBP1-/- mice surviving by day 13 after whole body X-irradiation to a dose of 500 cGy. Administration of a single dose of liposomes containing purified recombinant human RALBP1 (RALBP1-liposomes) i.p. caused detectable levels of RALBP1 in all tissues examined including brain 48 h after administration of liposomes. Three doses of liposomes caused further accumulation of RALBP1 in all tissues. Whole body irradiation to 500 cGy in RALBP1-/- mice administered control liposomes i.p. at day -3, +3 and +5 with respect to time of irradiation resulted in 0/6 animals surviving after 13 days. In marked contrast, 6/6 RALBP1-/- mice treated with the RALBP1-liposomes survived after 44 days. These remarkable results as well as our published and preliminary studies in cultured cell lead us to hypothesize that RALBP1 provides protection to cells from diverse stresses including heat-shock, oxidants, chemotherapeutic agents, UV-irradiation and X-irradiation and lead us to propose that RALBP1 is a multifunctional transporter and signaling protein functioning as a xenobiotic/oxidative-stress/radiation defense mechanisrn. We propose to test our hypothesis in the RIP1 (mouse ortholog of RALBP1) knockout mice we have obtained from Lexicon Genetics. We will perform studies with specific aims addressing the following hypotheses: 1) RALBP1 knockout will result in sensitization to the toxicity of X-irradiation; 2) RALBP1 knockout will result in sensitization to xenobiotic toxins including alkylating agents and natural product chemotherapy agents; 3) The transport activity of RALBP1 is regulated by POB1, a RAL-binding protein which causes apoptosis in prostate cancer cells when over-expressed. Towards these aims, we have designed detailed toxicological, biochemical, genetic and mechanistic studies proposed in the present application. Results of these studies will lead to the recognition of RALBP1 as a major xenobiotic defense mechanism and advance the potential use of RALBP1-liposomes as pharmacological agent for use in humans as a defense from the toxicity of xenobiotics and radiation.